This application claims the priority of prior German application 101 41 047.6, filed Aug. 22, 2001 the disclosure of which is expressly incorporated by reference herein.
The invention relates to a mirror, particularly a vehicle rear-view mirror, having a light-transmitting filtering layer for filtering out interfering fractions of the light entering the filtering layer in the yellow spectral range.
The spectral sensitivity of the human eye, that is, the sensitivity to light of different wavelengths, depends on the radiant flux density of the light impinging on the eye. At low radiant flux densities (night vision), the spectral maximum of the sensitivity curve is situated at smaller wavelengths than in light conditions during a normal bright day (day vision). Nevertheless, even in night vision, the eye is sensitive to light in the yellow spectral range.
It is known that light in the yellow spectral range has an interfering effect on the visual perception. This applies particularly to the visual perception of driving situations by means of a vehicle rear-view mirror when driving a motor vehicle at night or under similar light conditions.
U.S. Pat. No. 5,844,721 explains this phenomenon in detail. This document also describes a rear-view mirror with a light-transmitting glass layer which, for filtering out yellow light, contains neodymium oxide of a weight fraction of 5 to 20 percent. The thickness of the glass layer amounts to from 0.5 to 4 mm. According to U.S. Pat. No. 5,844,721, in this manner, 95 to 98 percent of the light with wavelengths of between 565 and 595 nm can be filtered out when the glass layer has a thickness of 3.39 mm. Viewed from the mirror surface, on which the light to be reflected is impinging, a reflection layer made of silver is arranged behind the glass layer for reflecting the light which has impinged on the mirror and has passed through the glass layer.
The disadvantage of such a mirror is the thickness of the glass layer of at least 0.5 mm. Particularly in the edge area of the mirror, such thicknesses result in blurred contours and/or double contours of the mirror images. Furthermore, the expenditures for the production of thick glass layers are relatively high.
It is an object of the present invention to expand the possibilities for producing a mirror of the initially mentioned type.
An important idea of the present invention is the combination of a light-transmitting filtering layer for filtering out interfering fractions of the light entering the filtering layer in the yellow spectral range by means of an interference reflector, the interference reflector having a plurality of thin layers for the reflection of the light impinging on the mirror, and the degree of reflection of the interference reflector in the yellow spectral range being lower than in an adjoining wavelength range with smaller wavelengths. The degree of reflection of the interference reflector in the yellow spectral range is preferably lower than in the entire green spectral range.
In a contemplated embodiment, in the wavelength range from 450 to 550 nm, particularly between 480 and 520 nm, the interference reflector has a degree of reflection maximum, and the degree of reflection decreases starting at the maximum to beyond the yellow spectral range. This can be achieved, for example, by a suitable coordination and selection of the number and/or the materials and/or of the layer thicknesses of the thin layers of the interference reflector. The layers are considered thin particularly when the light impinging on the interference reflector, because of the small layer thickness, is absorbed only to a negligibly small fraction, so that the sum of the degree of reflection and of the degree of transmission for each wavelength of the visible light amounts to approximately 1. Such interference layers typically have a layer thickness which is clearly smaller than the wavelength of the visible light. For example, the optical thickness of at least one of the interference layers amounts to one fourth of a wavelength of light in the range from 460 to 540 nm, particularly one fourth of the wavelength at which the reflection degree maximum is situated. Such layers can be easily reproduced by means of processes known per se, such as sputtering processes or thermal evaporation processes and can be deposited on a substrate while the layer thickness is uniform.
An advantage of the present invention relates to the fact that the endeavored extinction of the interfering fractions in the yellow spectral range does not have to be achieved only by an absorption in the filtering layer. On the contrary, the interference reflector contributes to a mirror image which can be easily perceived also under poor light conditions in that it reflects selectively. As a result, the layer thickness of the filtering layer can be reduced in comparison to the glass layer described in U.S. Pat. No. 5,844,721. In a preferred embodiment, the filtering layer therefore has a layer thickness of less than 0.5 mm. Particularly when the filtering layer is a glass layer, the manufacturing expenditures are therefore reduced. In contrast to thick glass layers, thin glass layers can be cut by means of customary laser tools. Also, in the case of laser-cut glass layers, an otherwise required beveling of the edge can be eliminated because the cut edges can be constructed correspondingly. In addition, mirror layers which are plane at first are frequently deformed in the further course of the manufacturing process in order to produce a mirror with a curved surface. The deforming of thin glass layers requires significantly less energy and lower expenditures. For example, a thin glass layer can be bent such that it adapts to the contour of a mold which adjoins on one side and which is used for the deformation. The mold can be used for heating the glass layer. After the cooling, the glass layer will then retain its curvature.
However, the invention is not limited to the use of glass layers as filtering layers. For example, as an alternative or in addition, a transparent plastic material, such as a suitable silicone rubber, can be used as the basic material for the filtering layer. In addition to the filtering-out of the interfering light fractions, the plastic material can also take over additional functions of the mirror, for example, the holding of additional mirror layers or the connecting of materials to opposite sides of the plastic layer and/or the sealing-off against a penetration of air and/or moisture.
In a further development, the mirror has an absorption layer for absorbing light which has penetrated the interference reflector, the absorption layer in the visible wavelength range having an average degree of reflection of less than 0.08, particularly 0.03 to 0.05. The absorption layer therefore absorbs the largest fraction of the impinging light and largely prevents a reflection of the yellow light not reflected by the interference reflector. Particularly when the degree of reflection of the interference reflector in the wavelength range of the green and/or red light is clearly lower than 1, by absorbing red or green light, the absorption layer contributes to the fact that an observer of the mirror receives an easily perceptible image. The reason is that, if these fractions were reflected, when the reflection is directed, blurry contours and/or multiple contours could occur for the observer. In the case of a diffuse reflection, the contrast of the light reflected by the interference reflector would not be sufficiently pronounced in comparison to the light reflected in the background.
In particular, the interference reflector has only three thin layers, for example, a center layer which is embedded between two outer layers, the outer layers having a higher refractive index than the embedded center layer. The center layer may be a silicon oxide layer, particularly with SiO2. Although, when only three thin layers are used, this does not result in the same design possibilities of the reflection behavior as when more layers are used, because of the absorption layer, the contrast is sufficiently pronounced also in the case of degrees of reflection clearly lower than 1 which can typically be achieved by means of three layers.
In a particularly preferred further development, the mirror has an illumination device by means of which the mirror background can be illuminated which is defined by the combination of the filtering layer and the interference reflector, so that light generated by the illumination device exits from the mirror surface. The mirror preferably has a control device for controlling the luminosity of the illumination device. Thus, the contrast of the light reflected by the interference reflector with respect to the light coming out of the mirror background can be changed. The effect on a viewer of the mirror is comparable to a change of the intensity of the light reflected by the interference reflector. This dimming effect can therefore be utilized for reducing the blinding effect of intensive light impinging on the mirror surface. For example, the intensive light is detected and the contrast is then reduced. Particularly when used in street/highway traffic, the mirror can therefore increase driving safety.
The above-described absorption layer is preferably provided in combination with the illumination device, in which case, viewed from the mirror surface, the light exit of the illumination device is arranged behind the absorption layer and the absorption layer is transmitting at least for a small fraction of the light generated by the illumination device. In particular, in the visible wavelength range, the degree of transmission amounts to more than 0.1, preferably more than 0.25.
The invention will now be explained in the manner of an example by means of the attached drawing. However, it is not limited to the described examples and embodiments.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.